204 THE BOOK OF NATURE STUDY 



skeleton ; their walls are provided with a closely wound spiral 

 thickening which serves to keep them open and give elasticity 

 against external pressure. Every muscular contraction com- 

 presses more or fewer of these tubes and drives air out of them, 



while every relaxation 

 allows the tubes to 

 expand again and to 

 receive fresh supplies 

 of air in consequence 

 of the atmospheric 

 pressure outside. This 

 is what is accom- 

 plished by the above- 

 mentioned rhythmic 

 movements of the ab- 

 domen. Inasmuch as 

 the oxygen of the 

 air is thus carried 

 direct to the tissues, it 



FIG. 75. Tracheal tube of an insect. , , , , 



follows that the blood 



is to a large extent relieved of the function which in our- 

 selves and many other animals it possesses, namely, of con- 

 veying oxygen from special respiratory surfaces to other parts. 

 Hence it is not surprising to find that the blood system of 

 wasps and other insects is but feebly developed. The details of 

 the internal anatomy are, however, beyond the scope of this 

 work. 



Life-history and Social Economy. As already stated, the 

 population of a wasps' nest towards the end of the summer com- 

 prises the " queen " foundress of the society, workers and drones, 

 and in addition young virgin " queens " destined to continue 

 the race in the following year. In early autumn these young 

 " queens " leave the nest, mate with a drone, and thus fertilised 

 seek out sheltered quarters in which to pass the winter. The 

 remainder of the community perishes at the onset of winter. 

 Hibernating " queens " may often be found in curtains, straw- 

 thatched roofs and other similar situations, during the winter. In 

 this state the wings are brought down on to the ventral side of the 



